The present invention relates to a stylus for use with a capacity change detection type disc system which grasps as a subtle change in capacity an audio signal, a video signal or like information signal recorded densely in an information recording medium in the form of a disc, and reads it out as an electric signal to reproduce the informtion signal. More particularly, the present invention is concerned with a stylus of the capacity change detection type in which a slide surface forms an essential part of the stylus and is slidingly engagable with a track on a disc, which is formed as a geometrical arrangement of pits representative of information signals, and in which an electrode portion is provided on the stylus for picking up the information signals in the track when the stylus traces the track causing a variation of electrostatic capacitance between the electrode and the pits. Still more particularly, the present invention is concerned with a capacity change detection type stylus in which the electrode portion is formed in a diamond body of the stylus body itself by implanting or injection ions into the diamond body.
Various kinds of disc systems have recently been developed which commonly serve to read or reproduce out of a disc audio, video or like information signals which are recorded in a high density in the disc. They include systems which are known as the VHD (Video High-Density Disc) system, AHD (Audio High-Density Disc) system, CED (Capacitance Electronic Disc) system. In such disc systems, information signals such as audio or video signals are stored in a high density in tracks which are formed in a disc as a spiral or concentric strings of pits corresponding to the information signals. The information signals are grasped as changes in capacity and detected and reproduced as changes in an electric signal. In the so-called capacity change detection type disc system described, information signals densely recorded in tracks on a disc are detected by an electrode portion formed in a stylus as changes in capacity and, therefore, the resolution attainable with the electrode portion may be noticeably increased by designing the thickness of the electrode portion sufficiently small relative to the pit size in the direction of tracking. For this reason, the disc system of the type described has an advantage that information signals can be recorded and reproduced even if the revolving speed of a disc is lowered, as well as other various advantages, thereby attracting increasing attention as one of high density recording and playback systems for discs.
Referring to FIG. 1, a disc D and a stylus S in accordane with a disc system of the type described are shown in a relative position for reading informtion signals out of a track of the disc D. As shown, the disc D has numerous pits p, p, p, . . . formed in its surface in correspondence with information signals. The stylus S comprises a body portion 10 made of diamond, and a detecting electrode portion 20 disposited on the body portion 10. The disc D is rotated as indicated by the arrow X in FIG. 1.
In the above-described type of disc system, the stylus body 10 comprises integrally therewith a slide surface slidingly engagable with a track on the disc as previously described, and the electrode portion 20 which intersects the slide surface at its end portion whose thickness is not more than one half the shortest recording wavelength assigned to the disc or the pit size previously mentioned, that is, the electrode end portion forms only a fractional part of the slide surface. In this construction, the stylus S remains in sliding contact with the disc D, which is rotatable at a high speed, while information signals are read out of the disc D. It is therefore a primary requisite that the stylus S prevents the electrode portion 20 from flaking off of its body 10 while making contact with the pits and detecting information signals with a desirable carrier-to-noise (C/N) ratio.
Typical of prior art styluses of the type with which the present invention is concerned is one having the detecting electrode portion 20 which is a thin layer of a conductive material deposited on a diamond, which constitutes the stylus body 10, by vacuum evaporation or a metal sputtering process. A problem encountered with such a stylus is that, because the conductive material deposited on the stylus body 10 wears easily, difficulty is experienced in producing the stylus body 10 with a predetermined electrode configuration by grinding off the stylus body leaving the desired predeposited conductive layer on the desired facet, i.e. the electrode portion 20. It is also difficult to form the electrode portion 20 to an even thickness on the predetermined surface of the stylus body 10, resulting in scattering in the characteristics of the products. Additionally, the conductive layer on the stylus body 10 is eventually apt to flake off while the styplus S is contacting the pits on the disc surface.
Efforts have heretofore been made to provide a stylus which is free from the drawbacks inherent in a capacity change detection type stylus having the conventional structure. In fact, there has recently been proposed a stylus of the type described having a unique structure in which both the side surface engagable with a track or pits on a disc and the electrode portion responsive to a change in the electrostatic capacitance as described are formed integrally with the diamond structure of the stylus body, particularly a stylus having an electrode portion formed in the diamond itself by implanting ions therein (see Japanese Utility Model Publication No. 57-40422/1982 and Japanese Patent Laid-Open Publication No. 56-153545/1981 for example).
In detail, metal ions, semi-metal ions or non-metal ions are implanted into the material which constitutes the stylus body, i.e. diamond which consists of carbon atoms and serves as an insulator. Then, the implanted ions function as an acceptor or a doner or transforms the diamond bond into a graphite bond, allowing the diamond converted to graphite to exhibit conductivity. Utilizing such a phenomenon, the proposed stylus implants ions into the diamond of a stylus body to form a conductive layer and uses this layer as an electrode portion responsive to a change in capacity.
When a dose of 10.sup.15 ions per square centimeters is implanted by a single injection into the diamond, the diamond shows a graphitizing rate as shown in FIG. 2, for example, in the depthwise direction from its surface. The graphitized part (graphitized layer) of the diamond is lowered in specific resistance down to about 10.sup.-3 ohm centimeters. Therefore, it is possible to form integrally with the diamond a detecting electrode portion consisting of a graphitized layer (hereinafter, sometimes referred to as a conductive layer consisting of a graphitized layer or simply as a conductive layer) by implanting ions into the diamond.
Also, the conductive layer developing in the diamond due to ion implantation is contollable in electric resistance, mechanical strength and the like by varying the implantation conditions. Furthermore, the implantation depth and the ion density profile are reproducible with considerable accuracy so that the electrode portion comprising a conductive layer prepared by ion implantation as described allows less scattering and attains higher mechanical resistance than the prior art electrode structure in which a conductive layer is deposited on a diamond. It will thus be seen that the stylus having an electrode portion formed by ion implantation is more desirable in various respects than the typical prior art stylus of the same type.
Now, the electric resistance and the mechanical strength of the conductive layer consisting of the graphitized layer developed in a diamond by ion implantation vary in matching relation with the graphitizing rate of the graphitized layer. Therefore, in the proposed stylus of the type described, the conductive layer provided by the single dose implantation undergoes a variation in graphitizing rate as shown in FIG. 2. In FIG. 2, toward deeper portions the graphitizing rate increases to exhibit a maximum at a portion of the diamond a little deeper than the surface of the diamond which ions hit first and penetrate (i.e. the surface of the electrode portion), then the graphitizing rate sharply decreasing from that portion deeper into the diamond as represented by an approximated straight line K1. This causes a sharp change in the depthwise direction to occur in the mechanical strength at the border of the electrode portion, the graphitizing rate of which sharply changes in the depthwise direction from the surface of the electrode portion. One way of determining such graphitizing rate is observing a color change of the diamond material under a scanning electron microscope. It has been studied and discovered by the inventor that while the stylus is kept in sliding contact with a moving disc for playback, the electrode portion tends to flake off the stylus body at the border where the sharp change in mechanical strength occurs. This is concluded from the fact that due to the low graphitizing rate on the surface of the electrode portion of the stylus, stresses concentrate at the sharp corner of the side surface and the electrode portion which is less susceptive to wear but transmits the stress further up to other parts of the electrode.
FIG. 3 is a fragmentary enlarged side elevation of the stylus S for illustrating the problem discussed above. In FIG. 3, the concentration of stresses occur at a sharp corner 22 of the electrode portion 20. The graphitizing rate sharply changes at the specific part of the electrode portion 20 of the stylus S which is designated by the reference numeral 24. The phantom line in FIG. 3 represents part of the electrode portion 20 which was removed due to flaking off.
The present invention has been elaborated to solve the various problems discussed hereinabove in conjunction with a prior art stylus of the capacity change detection type.